Closure of Ion-Pressure-Driven Magnetospheric Electric Currents Through the
Ionosphere:  Implications for Global Electric Fields

Edmond C. Roelof, Pontus C:son Brandt, Robert Demajistre, Donald G. Mitchell,
and Brian J. Anderson

Johns Hopkins University/Applied Physics Laboratory, 11100 Johns Hopkins
Road, Laurel, MD 20723 USA


Energetic neutral atom (ENA) images of H+ and O+ ions 10-60 keV/nuc from the
HENA camera on the IMAGE spacecraft reveal that the storm-time ring current
is often asymmetric.  This implies an azimuthal gradient in the ion pressure
that will drive field-aligned currents into and out of the ionosphere. 
Kinetic modeling of the ring current indicates that the closure of this
"partial ring current" through the ionosphere will in turn produce electric
fields in the magnetosphere.   Such electric fields are strong and are
distorted from the classical picture of just a solar wind plus corotational
electric field.  This in turn implies that the transport of ring current ions
may be dominated by localized regions of fast ExB drifts, as is required to
explain the temporal and spatial dependence of the asymmetry observed in the
HENA images.  We are now able to calculate the global electric currents
throughout the magnetosphere that are driven by the ion pressure gradients. 
Where those currents enter the ionosphere, we can calculate the intensity of
the Region 2 field-aligned current (FAC) system.  The ion pressure (P) is
deduced from the H+ and O+ ion intensities extracted by inversion of the HENA
images (assuming pitch-angle isotropy).  Then the electric current intensity
(J) may be calculated from the force-balance relation JxB=P using the Euler 
potential representation of J introduced by Roelof [Adv. Space Res., 9(12), 
195, 1989].  For storm main phases (e.g., 4 October 2000), the computed FAC 
into the ionosphere has been compared with the FAC measured directly by 
magnetometers on the Iridium low-altitude satellites. We have also analyzed 
periods during which the partial ring current builds up, and we find a 
consistent increase in the FAC pattern.  We expect that further aspects of 
this kind of magnetosphere-ionosphere coupling may be revealed by comparison 
with EUV images from the IMAGE spacecraft of plasmasphere erosion in response 
to the electric fields that are generated by this process. 

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Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002